Intellectual property agreement – what document?

Location: IP owner #1 – EU (me); IP owner #2 – Canada (co-author); Company – US

I am a videogame developer who co-authored a piece of intellectual property (3d artwork) for a videogame alongside a co-author. A third party, which is a company that owns a videogame for which this artwork was developed, then takes our intellectual property distributing it to it’s subscribers/clients while paying us out what I assume in the US (where the company is based) is regarded as royalties. It is up to us, the two authors, to negotiate between ourselves what percentage of the payout we individually get from the sum that the company pays us out. We then simply provide the figure to the company in writing and they wire the money to our banks.

I am in a position where I am effectively in complete control of everything about this intellectual property against the other co-author. He was previously rewarded with a very favorable split, but going forward, because of developments with this IP, I want to decrease his royalty percentage while increasing mine.

Is there some sort of document I could present my co-author with where we get the ownership percentages of this IP in writing in some simple manner? The goal is simply to make it look serious as the other party has no legal knowledge on the topic either, but if it has any actual legal power it would be even better. Sorry for the vagueness.